Effect of Number of Fusion Levels on S1 Screws in Long Fusion Construct in a Calf Spine Model

Abstract

In vitro biomechanical study evaluating the effect of fusion levels on simulated S1 screws on calf spines. To measure the stresses of simulated S1 screws as a function of increasing fusion levels in long-fusion construct. Extension of long fusion to sacrum remains a difficult clinical challenge despite the recent knowledge and technology. Several biomechanical studies have evaluated the bending moment on sacral screw of various fixation techniques. The high risk of sacral screw failure has been primarily attributed to the long lever arm of instrumentation and pullout force on S1 screw. However, none of these studies characterized how the number of fusion levels affected the sacral screw stresses. Because the number of fusion levels is an important factor in considering additional fixation options to protect the sacral screws, the effect of fusion levels on sacral screw stresses must be understood. Eight calf spines (T13-L5) were used in this study (L5 is simulated as S1). Twelve pedicle screws were inserted. A combination of strain gages were attached to the spinal rods located between L4 and L5. Pure flexion moment (7.5 Nm) was applied to the partially fused construct as the level of fusion varied between 1 and 5 levels. The number of fusion levels increased from lower vertebrae to upper vertebrae. The pullout force, bending moment, and transverse force acting on the S1 screw were calculated from the strain gage signals. One way repeated measure ANOVA was used to analyze the pullout force, bending moment, and transverse force data. The pullout force and transverse load had significant difference between single and multilevel fusion. However, no difference was found among multilevel fusion procedures (all P > 0.05). The mean pullout force was 29 ± 8 and mean transverse force was 139 ± 18 N. For bending moment, there was no statistical difference among all fusion levels. The average bending moment was 1.02 ± 0.34. Addition of anterior interbody cages at lower levels (L3-L4 and L4-L5) reduced the stresses on the simulated S1 screw by approximately 20% with P < 0.05. All three stresses, flexion moment, transverse load, and screw pullout force, must be considered in studying the biomechanics of long-fusion construct. There was no significant increase in sacral screw loading with increasing fusion levels when the same flexion moment was applied. It is postulated that the higher load present in long-fusion construct may lead to the premature failure of the lumbosacral fixation.